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STRUCTURE OF CLOVANEMAGNOLOL, A NOVEL NEUROTROPHIC SESQUITERPENE-NEOLIGNAN

FROM MAGNOLIA OBOVATA

* Yukio Otoshi, *

Yoshiyasu Fukuyama, and Mitsuaki Kodama,

Faculty of Pharmaceutical Sciences, Tokushima Bunri University,

Yamashiro-cho, Tokushima 770, Japan

Takashi Hasegawa and Hiroshi Okazaki

Otsuka Pharmaceutical Co., Ltd., Kagasuno, Tokushima 771-01, Japan

ABSTRACT: The structure of novel sesquiterpene-neolignan, clovanemagpolol (11,

isolated from the bark of Magnolia obovata has been elucidated on the basis of

the extensive ZD-NMR analyses and finally substantiated by one-step biomimetic

synthesis from caryophyllene oxide and magnolol. Clovanemagnolol has exhibit-

ed neurotrophic activity at 10e7 M on neuronal cell culture system of fetal

rat cerebral hemisphere.

In our studies on the minor constituents in the methanol extract of the

dried bark of medicinal plant Magnolia obovata THUNB., we have to date

successfully isolated a number of novel sesquiteprene-neolignans, eudesobovat-

01 A, eudesobovatol B,') eudesmagnolol, and eudeshonokiol A, 2) which are

featured to possess both of eudesmane-type sesquiterpene and biphenyl-type

neolignan moieties in the structure. Among them, eudesobovatol A3) was

revealed to have an NGF-like neurotrophic action on cultured neuronal cell

derived from fetal rat cerebral hemisphere.4) We now report the structure of

more potent neurotrophic active sesquiterpene-neolignan, clovanemagnolol (11,

with rboth units of tricyclic sesquiterpene clovanediol (2)5) and neolignan

magnolol (4j6) in a molecule.

2

la: R = AC

4478

Clovanemagnolol (l)7) had the molecular formula of C33H4203 [m/z 486.3127

(M+)] in HHMS and its 'Ii NME spectruma) indicated the presence of an aromatic

and sesquiterpene moieties. The aromatic part was assigned as magnolol based

on the occurrence of two sets of a spin system typical of an ally1 group and

a 1,2,4_trisubstituted aromatic ring in 'Ii NMB and comparison of 13C NME

data') with those of magnolol as well as on the observation of a prominent

fragment ion peak at m/z 266 in the EIMS. On the other hand, the spectral

data of the terpene part were totally different from those of eudesmol-type

structure existing in all the sesquiterpene-neolignans previously reported.'g2)

The DEPT spectrum of 1 showed the presence of fifteen carbons consisted of

three methyl, six methylene, one methine, two oxygen bearing methine, and

three quarternary carbons, whereas its 'Ii NME spectrum contained three

tertiary methyl signals (6 0.64, 0.84, 0.94) and two carbinyl methine signals

[d 3.07 (d, J = 3.2, 3.2 Hz), 4.11 (dd, J = 8.5, 5.8 Hz)]. Acetylation of 1

gave diacetate la, and thereby the only methine signal at 6 3.01 in 1

largely down-field shifted to 6 4.49 in the case of la indicating the high-

field and low-field carbinyl methine protons attached on carbons bearing a

hydroxyl group and an ether oxygen atom, respectively. Moreover, it was

definitely shown by the ZD-NME experiments (DQF-COSY and C/H-COSY) that these

two carbinyl methine protons were involved in partial structures A and B,

respectively, and also the additional connectivities C and D were present in 1.

Fig. Dotted lines indicate the

connectivities of units A-D

inferred from H/H and C/H COSY.

Arrows denote the correlation

between protons (tail) and

carbons (head) around the

quarternary carbons observed

in the HMBC.

The above partial structures were eventually assembled as shown in Fig. based

on two and three bonds cross peaks between the proton signals and the carbon

signals around the three remaining quarternary carbons (6 34.8, 37.8, and

45.3) observed in HMBC. The spectral evidence aforementioned disclosed that

the sesquiterpene moiety should be corresponding to a tricyclic clovane-type

skeleton.g) The NOE enhancement of ortho-coupled aromatic resonance at 6 6.95

(d, J = 8.3 Hz, H-3') upon irradiation of the methine signal at 6 4.11 allowed

us to get the both magnolol and sesquiterpene moieties assembled together

through an ether bond between the C-2 position in clovane and the one hydroxy

group in magnolol. Thus, the proposed tentative structure 1 is most likely to

4479

be formed by attack of magnolol as a nucleophile to the carbonium ion

generated in the course of well known transannular cyclization of caryophyll-

ene, since analogous clovanediol (2) has been already verified to be

converted from caryophyllene oxide by acid-catalyzed rearrangement.") With

this anticipation in mind, one step synthesis of 1 was attempted. Treatment

of a mixture of (-)-caryophyllene B-oxide (3) and magnolol in anhydrous ether

with one drop of cont. H2SOq gave rise to an addition product ([aID 26.3)

whose 'H and 13C NMH data were superimposable with those of 1. This

successful synthesis led to the straightforward conclusion that clovanemagnol-

01 must have the structure including absolute stereochemistry as shown in 1 in

the light of the already established mechanistic aspects on a series of

Wagner-Meerwein-type skeletal rearrangement of caryophyllene.lO~ll). In fact,

the H-9 has small J values (dd, J = 3.2, 3.2 Hz) indicating a-axial

orientation of the Cg-OH and ring B in 1 has to adopt the boat form like that

of clovanediol (2)'O) owing to not only large J values (dd, J = 11.5, 5.6 Hz)

for the H-5 but also the NOE interaction between the H-6u and the C4u-CH3. These results, thus, fully corroborated the structure 1 for clovanemagnolol.

I HI __

I

-P HI'

H+/ether Hb OH

3

Clovanemagnolol (I) is unique in having clovane-type sesquiterpene which

1

has not been served as a counterpart in the previously reported sesquiterpene-

neolignans, and one step synthesis of 1 could be realized from (-)-

caryophyllene 6-epoxide and magnolol based on its biogenetic consideration.

Finally, it should be emphasized that clovanemagnolol (1) can not only

accelerate neurite sprouting and neuronal cell network formation but also

greatly enhance choline acetyltransferase activity12) at 10m7 M on cultured

neuronal cell of fetal rat cerebral hemisphere and is so far the most potent

neurotrophic substance amongist the active substnces found in N. obovata.

Acknowledgement We thank Dr. H. Iwabuchi (Sanei Chemical Co. Ltd.,)

for kind gift of the NMH data of clovene and clovanol.

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References and Notes

1. Y. Fukuyama, Y. Otoshi, M. Kodama, T. Hasegawa, Ii. Okazaki, and M. Nagasawa,

Tetrahedron Letters, 30, 5907 (1989).

2. Y. Fukuyama, Y. Otoshi, K. Nakamura, M. Kodama, M. Sugawara, and M.

Nagasawa, Chem. Lett.. 1990, 295.

3. A part of this study was presented at the 1989 International Chemical

Congress of Pacific Basin Societies, Honolulu, 1989.

4. H. ASOU, N. Isasaki, S. Hirano, and D. Dahl, Brain Research. 332, 355 (1985).

5. A. S. Gupta and S. Dev, Tetrahedron, 27, 635 (1971).

6. Y. Sugi, Yakugaku Zasshi, 50, I83 (1930).

7. 1: [aID 21.0° (c 1.5, CHC13); W (EtOH): 204 (E 4600), 208 (E 41000), 286

(E 5800) nm; IR (CHC13): 3550 and 3350 (OH), 1640 (C=C), 1500 (aroma.)

cm -1 ; EIMS s/Z (rel. int.): 486.3127 (Ml+ (7, calcd. 486.3134 for

C33H4203)r 266 (loo), 223 (13), 221 (5), 203 (10).

8. 'H NMR (400 MHz, C6D6): 6 0.64 (3H, s, H-13), 0.68 (lH, d, J = 13.9 Hz,

H-l Za), 0.83 (IH, m, H-78), 0.84 (3H, s, H-14), 0.94 (3H, sI H-151, I-02

(IH, dddd, J = 11.5, 11.5, 11.0, 6.1 Hz, H-6a), 1.08 (lH, ddd, J = 13.6,

3.2, 3.2 Hz, H-118), 1.12 (lH, m, H-68), 1.15 (IH, m, H-'la), 1.24 (IH, dd,

J = 11.5, 5.6 Hz, H-5), 1.38 (IH, dddd, J = 13.6, 3.2, 3.2, 3.2 Hz, H-lOa),

1.50 (lH, dd, J = 12.7, 8.5 Hz, H-38), I.56 (IH, d, J = 13.9 Hz, H-128),

1.59 (lo, dd, J = 12.7, 5.8 Hz, H-3a), 1.68 (lH, ddd, J = 13.6, 13.6, 3.2

Hz, H-108), 1.77 (lo, dddd, J = 13.6, 13.6, 3.2, 3.2 HZ, H-llu), 3.07 (IH,

dd, J = 3.2, 3.2 Hz, H-98), 3.19 (2H, d, J = 6.8 Hz, H-7'), 3.24 (lH, d, J

= 6.6 Hz, H-7"), 4.11 (lo, dd, J = 8.5, 5.8 Hz, H-2a), 4.97 (IH, dd, J =

16.9, I.2 Hz, H-9'), 4.99 (lH, dd, J = 10.0, I,2 Hz, H-9'), 5.00 (IH, dd,

J = 10.0, I.2 Hz, H-9"), 5.02 (lH, dd, J = 16.8, I.2 Hz, H-9"), 5.88 (IH,

ddt, J = 16.9, 10.0, 6.8 Hz, H-8'), 5.94 (lH, ddt, J = 16.8, 10.0, 6.6 Hz,

H-8"), 6.95 (lH, d, J = 8.3 Hz, H-3'), 7.04 (lH, dd, J = 8.3, 2.2 Hz,

H-4"), 7.06 (IH, dd, J = 8.3, 2.4 HZ, H-4'), 7.17 (lH, d, J = 2.4 Hz,

H-6'), 7.18 (lH, d, J = 2.2 Hz, H-6"), 7.20 (IH, d, J = 8.3 Hz, H-3");

13C NMR (100 MHz, 21.0 (C-6), 25.5 (C-13), 26.8 (C-IO), 27.1 C6D6):6

(C-11), 28.7 (C-151, 31.3 (C-141, 33.2 (C-71, 34.8 (C-81, 35.7 (C-12),

37.8 (C-4), 39.7 (C-7', 7"), 44.5 (C-3), 45.3 (C-l), 50.3 (C-5), 74.7

(C-g), 89.7 (c-2), 115.3 (C-g"), 115.8 (C-9'), 116.3 (C-3'), 117.6 (C-3"),

127.4 (c-l"), 129.2 (C-4'), 129.5 (C-l'), 129.7 (C-4"), 131.6 (C-6"),

132.1(C-5"), 132.8 (C-6'), 134.1 (C-5'), 137.8 (C-8'), 138.4 (C-8"),

153.1 (C-2"), 154.6 (C-2').

9. A. w. Lutz and E. B. Reid, J. Chem. Sot., 1954, 2265.

10. A. Aebi, D. H. R. Barton, and A. S. Lindsey, J. Chem. Sot., 1953, 3124.

Il. F. w. Mckillop, J. Martin, W. Parker, J. S. Roberts, and J. R. Stevenson,

J. Chem. Sot., (Cl, 1971, 3375.

12. F. Fonnum, J. Neurochemistry, 24, 407 (1975).

(ReceivedinJap~ 19Apfil1990)